entity_language_fallback.module

<?php

/**
 * @file
 * Add fallback languages to entities.
 */

use Drupal\Core\Form\FormStateInterface;
use Drupal\language\ConfigurableLanguageInterface;
use Drupal\language\Entity\ConfigurableLanguage;

/**
 * Implements hook_language_fallback_candidates_alter()
 */
function entity_language_fallback_language_fallback_candidates_alter(array &$candidates, array $context) {
  $operation = $context['operation'];

  if ($operation == 'entity_upcast' || $operation == 'entity_view') {
    $attempted_langcode = $context['langcode'];
    /** @var @var \Drupal\node\NodeInterface $entity */
    $entity = $context['data'];

    if (is_a($entity, 'Drupal\node\NodeInterface')) {
      $candidates = [$attempted_langcode => $attempted_langcode];
      /** @var Drupal\language\Entity\ConfigurableLanguage $language */
      $language = ConfigurableLanguage::load($attempted_langcode);
      $fallback_langcodes = $language->getThirdpartySetting('entity_language_fallback', 'fallback_langcodes', []);

      foreach ($fallback_langcodes as $fallback_langcode) {
        if (!empty($fallback_langcode) && $attempted_langcode != $fallback_langcode && !in_array($fallback_langcode, $candidates)) {
          $candidates[$fallback_langcode] = $fallback_langcode;
        }
      }
    }
  }
}

/**
 * Implements hook_form_FORM_ID_alter()
 */
function entity_language_fallback_form_language_admin_edit_form_alter(&$form, FormStateInterface $form_state) {
  /** @var Drupal\language\Entity\ConfigurableLanguage $this_language */
  $this_language = $form_state->getFormObject()->getEntity();

  $languages = Drupal::languageManager()->getLanguages();
  $options = [];
  foreach($languages as $language) {
    // Only include this language if its not itself.
    if ($language->getId() != $this_language->getId()) {
      $options[$language->getId()] = $language->getName();
    }
  }

  $form['entity_language_fallback'] = [
    '#title' => t('Entity fallback language'),
    '#description' => t('Choose one or more fallback languages in prioritized order. The languages are used as fallback in entity view.'),
    '#type' => 'details',
    '#open' => TRUE,
    '#tree' => TRUE,
  ];

  // Creating one priority field per available language.
  $default_values = $this_language->getThirdPartySetting('entity_language_fallback', 'fallback_langcodes', []);
  for ($i = 0; $i < count($options); $i++) {
    $form['entity_language_fallback'][$i] = [
      '#type' => 'select',
      '#title' => t('Priority @priority', ['@priority' => $i + 1]),
      '#description' => t('Choose the language used as priority @priority fallback language.', ['@priority' => $i + 1]),
      '#options' => $options,
      '#default_value' => !empty($default_values[$i]) ? $default_values[$i] : '',
      '#empty_option' => t('-None-'),
      '#tree' => TRUE,
    ];
  }

  $form['#entity_builders'][] = 'entity_language_fallback_form_language_admin_edit_form_builder';
}

/**
 * Entity builder for the language form entity_language_fallback options.
 *
 * @see entity_language_fallback_form_language_admin_edit_form_alter()
 */
function entity_language_fallback_form_language_admin_edit_form_builder($entity_type, ConfigurableLanguageInterface $this_language, &$form, FormStateInterface $form_state) {
  $this_language->setThirdPartySetting(
    'entity_language_fallback',
    'fallback_langcodes',
    $form_state->getValue('entity_language_fallback')
  );
}